Influence of cross-section geometry and wire orientation on the phonon shifts in ultra-scaled Si nanowires

Engineering of the cross-section shape and size of ultra-scaled Si nanowires (SiNWs) provides an attractive way for tuning their structural properties. The acoustic and optical phonon shifts of the free-standing circular, hexagonal, square and triangular SiNWs are calculated using a Modified Valence Force Field (MVFF) model. The acoustic phonon blue shift (acoustic hardening) and the optical phonon red shift (optical softening) show a strong dependence on the cross-section shape and size of the SiNWs. The triangular SiNWs have the least structural symmetry as revealed by the splitting of the degenerate flexural phonon modes and The show the minimum acoustic hardening and the maximum optical hardening. The acoustic hardening, in all SiNWs, is attributed to the decreasing difference in the vibrational energy distribution between the inner and the surface atoms with decreasing cross-section size. The optical softening is attributed to the reduced phonon group velocity and the localization of the vibrational energy density on the inner atoms. While the acoustic phonon shift shows a strong wire orientation dependence, the optical phonon softening is independent of wire orientation.Comment: 10 figures, 4 Tables, submitted to JAP for revie

Atomistic approach to alloy scattering in Si1−xGexSi_{1-x}Ge_{x}

SiGe alloy scattering is of significant importance with the introduction of strained layers and SiGe channels into CMOS technology. However, alloy scattering has till now been treated in an empirical fashion with a fitting parameter. We present a theoretical model within the atomistic tight-binding representation for treating alloy scattering in SiGe. This approach puts the scattering model on a solid atomistic footing with physical insights. The approach is shown to inherently capture the bulk alloy scattering potential parameters for both n-type and p-type carriers and matches experimental mobility data.Comment: 3 pg 3 figure

Full 3D Quantum Transport Simulation of Atomistic Interface Roughness in Silicon Nanowire FETs

The influence of interface roughness scattering (IRS) on the performances of silicon nanowire field-effect transistors (NWFETs) is numerically investigated using a full 3D quantum transport simulator based on the atomistic sp3d5s* tight-binding model. The interface between the silicon and the silicon dioxide layers is generated in a real-space atomistic representation using an experimentally derived autocovariance function (ACVF). The oxide layer is modeled in the virtual crystal approximation (VCA) using fictitious SiO2 atoms. -oriented nanowires with different diameters and randomly generated surface configurations are studied. The experimentally observed ON-current and the threshold voltage is quantitatively captured by the simulation model. The mobility reduction due to IRS is studied through a qualitative comparison of the simulation results with the experimental results

Atomistic study of electronic structure of PbSe nanowires

Lead Selenide (PbSe) is an attractive `IV-VI' semiconductor material to design optical sensors, lasers and thermoelectric devices. Improved fabrication of PbSe nanowires (NWs) enables the utilization of low dimensional quantum effects. The effect of cross-section size (W) and channel orientation on the bandstructure of PbSe NWs is studied using an 18 band $sp^3d^5$ tight-binding theory. The bandgap increases almost with the inverse of the W for all the orientations indicating a weak symmetry dependence. [111] and [110] NWs show higher ballistic conductance for the conduction and valence band compared to [100] NWs due to the significant splitting of the projected L-valleys in [100] NWs.Comment: 4 figures, Prepared for AP

Equatorial scintillations in relation to the development of ionization anomaly

International audienceThe irregularities in the electron density distribution of the ionosphere over the equatorial region frequently disrupt space-based communication and navigation links by causing severe amplitude and phase scintillations of signals. Development of a specification and forecast system for scintillations is needed in view of the increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillations. It has been suggested in recent years that a developed equatorial anomaly in the afternoon hours, with a steep gradient of the F-region ionization or Total Electron Content (TEC) in the region between the crest and the trough, may be taken as a precursor to scintillations on transionospheric links. Latitudinal gradient of TEC measured using Faraday Rotation technique from LEO NOAA 12/14 transmissions during the afternoon hours at Calcutta shows a highly significant association with L-band scintillations recorded on the INMARSAT link, also from Calcutta, during the equinoxes, August through October 2000, and February through April 2001. The daytime equatorial electrojet is believed to control the development of the equatorial anomaly and plays a crucial role in the subsequent development of F-region irregularities in the post-sunset hours. The diurnal maximum and integrated value (integrated from the time of onset of plasma influx to off-equatorial latitudes till local sunset) of the strength of the electrojet in the Indian longitude sector shows a significant association with post-sunset L-band scintillations recorded at Calcutta during the two equinoxes mentioned earlier. Generation of equatorial irregularities over the magnetic equator in the post-sunset hours is intimately related to the variation of the height of the F-layer around sunset. Ionosonde data from Kodaikanal, a station situated close to the magnetic equator, has been utilized to calculate the vertical drift of the F-layer over the magnetic equator for the period August through October 2000. The post-sunset F-region height rise over the magnetic equator shows a remarkable correspondence with the occurrence of scintillations at Calcutta located near the northern crest of the equatorial anomaly. Existence of a flat-topped ionization distribution over the magnetic equator around sunset has been suggested as a possible indication of occurrence of post-sunset scintillations. Width of the latitudinal distribution of ionization obtained from DMSP satellite shows some correspondence with post-sunset L-band scintillations. During the period of observation of the present study (August through October 2000, and February through April 2001), it has been observed that although the probability of occurrence of scintillations is high on days with flat-topped ion density variation over the equator, there are cases when no scintillations were observed even when a pronounced flat top variation was recorded

Bandstructure Effects in Silicon Nanowire Hole Transport

Bandstructure effects in PMOS transport of strongly quantized silicon nanowire field-effect-transistors (FET) in various transport orientations are examined. A 20-band sp3d5s* spin-orbit-coupled (SO) atomistic tight-binding model coupled to a self consistent Poisson solver is used for the valence band dispersion calculation. A ballistic FET model is used to evaluate the capacitance and current-voltage characteristics. The dispersion shapes and curvatures are strong functions of device size, lattice orientation, and bias, and cannot be described within the effective mass approximation. The anisotropy of the confinement mass in the different quantization directions can cause the charge to preferably accumulate in the (110) and secondly on the (112) rather than (100) surfaces, leading to significant charge distributions for different wire orientations. The total gate capacitance of the nanowire FET devices is, however, very similar for all wires in all the transport orientations investigated ([100], [110], [111]), and is degraded from the oxide capacitance by ~30%. The [111] and secondly the [110] oriented nanowires indicate highest carrier velocities and better ON-current performance compared to [100] wires. The dispersion features and quantization behavior, although a complicated function of physical and electrostatic confinement, can be explained at first order by looking at the anisotropic shape of the heavy-hole valence band.Comment: 30 pages, 7 figures, to be published in IEEE Transactions on Nanotechnolog

Bandstructure Effects in Silicon Nanowire Electron Transport

Bandstructure effects in the electronic transport of strongly quantized silicon nanowire field-effect-transistors (FET) in various transport orientations are examined. A 10-band sp3d5s* semi-empirical atomistic tight-binding model coupled to a self consistent Poisson solver is used for the dispersion calculation. A semi-classical, ballistic FET model is used to evaluate the current-voltage characteristics. It is found that the total gate capacitance is degraded from the oxide capacitance value by 30% for wires in all the considered transport orientations ([100], [110], [111]). Different wire directions primarily influence the carrier velocities, which mainly determine the relative performance differences, while the total charge difference is weakly affected. The velocities depend on the effective mass and degeneracy of the dispersions. The [110] and secondly the [100] oriented 3nm thick nanowires examined, indicate the best ON-current performance compared to [111] wires. The dispersion features are strong functions of quantization. Effects such as valley splitting can lift the degeneracies especially for wires with cross section sides below 3nm. The effective masses also change significantly with quantization, and change differently for different transport orientations. For the cases of [100] and [111] wires the masses increase with quantization, however, in the [110] case, the mass decreases. The mass variations can be explained from the non-parabolicities and anisotropies that reside in the first Brillouin zone of silicon.Comment: 35 pages, 7 figures, submitted to IEEE TE

Tuning Lattice Thermal Conductance in Ultra-Scaled Hollow SiNW: Role of Porosity Size, Density and Distribution

Porous crystalline Si nanowires (PC-SiNW) represent an attractive solution for enhancing the thermoelectric efficiency (ZT) of SiNWs by reducing the lattice thermal conductance (κl). A modified valence force field (MVFF) phonon model along with Landauer’s approach is used to analyze the ballistic κl in PC-SiNWs. A systematic study focusing on the influence of pore size, density, and distribution on the ballistic κl of PC-SiNWs is presented. The model predicts a maximum reduction of ~19%, ~23% and ~30% for 1, 2 and 3 pores, respectively with a constant removal of ~12% of the atoms in all the cases. The model also predicts a higher reduction of the ballistic κl as the pore separation increases, in the case of 2, 3 and 4 pores, for the same percentage of atoms removed (~12%) in all the cases. Thus, the presence of a high number of small, well-separated pores suppress κl strongly. This reduction in ballistic κl, in the coherent limit, is attributed to the reduction of the total number of phonon modes and smaller participation of phonon modes (in κl) with increasing number of pores

Intrinsic Reliability improvement in Biaxially Strained SiGe p-MOSFETs

In this letter we not only show improvement in the performance but also in the reliability of 30nm thick biaxially strained SiGe (20%Ge) channel on Si p-MOSFETs. Compared to Si channel, strained SiGe channel allows larger hole mobility ({\mu}h) in the transport direction and alleviates charge flow towards the gate oxide. {\mu}h enhancement by 40% in SiGe and 100% in Si-cap SiGe is observed compared to the Si hole universal mobility. A ~40% reduction in NBTI degradation, gate leakage and flicker noise (1/f) is observed which is attributed to a 4% increase in the hole-oxide barrier height ({\phi}) in SiGe. Similar field acceleration factor ({\Gamma}) for threshold voltage shift ({\Delta}VT) and increase in noise ({\Delta}SVG) in Si and SiGe suggests identical degradation mechanisms.Comment: 4 figures, 3 pages, accepted for publication in IEEE ED